Liquid crystal display devices have been applied to, for example, watches, calculators, a variety of household electrical appliances, measuring equipment, panels used in automobiles, word processors, electronic notebooks, printers, computers, and television sets. Representative examples of types of liquid crystal display devices include a TN (twisted nematic) type, an STN (super twisted nematic) type, a DS (dynamic scattering) type, a GH (guest-host) type, an IPS (in-plane switching) type, an OCB (optically compensated birefringence) type, an ECB (electrically controlled birefringence) type, a VA (vertical alignment) type, a CSH (color super homeotropic) type, and an FLC (ferroelectric liquid crystal) type. Regarding a drive system, multiplex driving has become popular instead of typical static driving; and an active matrix (AM) in which, for example, a TFT (thin film transistor) or a TFD (thin film diode) is used for driving has become standard rather than a passive matrix in recent years.
As illustrated in FIG. 1, in a general color liquid crystal display device, a transparent electrode layer (3a) as a common electrode and a color filter layer (2) are disposed between one of two substrates (1) and one of alignment films (4) provided so as to correspond thereto, a pixel electrode layer (3b) is disposed between the other substrate and the other alignment film, the substrates are disposed such that the alignment films face each other, and a liquid crystal layer (5) is disposed therebetween.
The color filter layer is a color filter consisting of a black matrix, a red layer (R), a green layer (G), a blue layer (B), and optionally a yellow layer (Y).
Impurities remaining in liquid crystal materials used in a liquid crystal layer have a large effect on the electrical properties of a display device, and the impurities have been therefore highly controlled. In terms of materials used in alignment films, it has been known that impurities remaining in the alignment films directly contacting a liquid crystal layer shift to the liquid crystal layer with the result that the impurities affect the electrical properties of the liquid crystal layer; hence, the relationship between the properties of liquid crystal display devices and impurities contained in the materials of alignment films have been being studied.
Also in terms of materials used in a color filter layer, such as organic pigments, it is believed that impurities contained therein have an effect on a liquid crystal layer as in the materials of alignment films. However, since an alignment film and a transparent electrode are disposed between the color filter layer and the liquid crystal layer, it has been believed that direct effect thereof on the liquid crystal layer is significantly smaller than that of the materials of the alignment film. In general, however, the thickness of the alignment film is only not more than 0.1 μm, and the thickness of the transparent electrode that is a common electrode disposed on the color filter layer side is not more than 0.5 μm even in the case where the thickness is increased to enhance the electric conductivity. Hence, the color filter layer and the liquid crystal layer are not in a state in which they are completely isolated from each other, and the color filter layer may therefore cause problems owing to impurities which are contained in the color filter layer and which pass through the alignment film and the transparent electrode, such as a decrease in the voltage holding ratio (VHR) of the liquid crystal layer and defective display including voids due to increased ion density (ID), uneven alignment, and screen burn-in.
Techniques for overcoming defective display caused by impurities present in a pigment contained in the color filter layer have been studied, such as a technique in which dissolution of impurities in liquid crystal is controlled by use of a pigment in which the amount of an extract from the pigment by ethyl formate is at a predetermined level or lower (Patent Literature 1) and a technique in which dissolution of impurities in liquid crystal is controlled by use of a specific pigment for a blue layer (Patent Literature 2). These techniques, however, are substantially not different from merely reducing the impurity content in a pigment and are insufficient in improvements to overcome defective display even in a current situation in which a technique for purifying pigments has been advanced.
In another disclosed technique, attention is paid to the relationship between organic impurities contained in a color filter layer and a liquid crystal composition, the degree in which the organic impurities are less likely to be dissolved in a liquid crystal layer is represented by the hydrophobic parameter of liquid crystal molecules contained in the liquid crystal layer, and the hydrophobic parameter is adjusted to be at a predetermined level or more; furthermore, since such a hydrophobic parameter has a correlation with a —OCF3 group present at an end of a liquid crystal molecule, a liquid crystal composition is prepared so as to contain a predetermined amount or more of a liquid crystal compound having a —OCF3 group at an end of each liquid crystal molecule thereof (Patent Literature 3).
Also in such disclosure, however, the technique is substantially for reducing effects of impurities present in a pigment on the liquid crystal layer, and a direct relationship between the properties of a colorant itself used in the color filter layer, such as a dye or a pigment, and the structure of a liquid crystal material is not considered; thus, problems of defective display in highly-developed liquid crystal display devices have not been overcome.